A neuropsychological profile of dementia was drafted for individuals with Alzheimer's Disease, Huntington's Disease and 'at risk' for Huntington's Disease. The evaluations extended into memory, learning and perceptual areas, utilizing standard and experimental tasks, also establishing normative references for functional changes accompanying the aging processes. Although Alzheimer's Disease is accompanied by marked deficits in selective attention, memory and learning, there were no qualitative differences between demented and age-matched subjects. The impairment also extended to object-naming and fluency, and AD patients performed poorly in perceiving meaning, except when the stimuli required emotional judgment. The data indicate that Alzheimer's patients may be unable to encode material; this is in sharp contrast with other amnesic disorders where the primary difficulty involves an inability to store and/or retrieve information. Alzheimer's and Huntington's patients showed pronounced but dissimilar deficits with visuospatial and constructional tasks. The behavioral data extend neuropathologic impressions of degeneration of the frontal striatal system in Huntington's Disease, and temporo-parietal, cortical involvement in Alzheimer's Disease. The neuropsychological test profile of Alzheimer's patients yielded different clinical subgroups or populations. Memory and learning deficits, per se, were poor indicators of group membership. One group was characterized by severely impaired verbal abilities, but with intact perceptual and constructional skills. The second group was more impaired on perceptuomotor than verbal tasks. The third group showed comparable deficiencies in both linguistic and visual spatial sectors. Positron emission tomographic and EEG data confirmed corresponding changes in left, right or bilateral regions in the posterior cerebral quadrant, respectively.